feat(users/sterni/nix/string): very simple printf implementation

This is mostly to yet another silly idea which turns out to be
possible. This may be actually useful should I implement more
sophisticated format specifiers like "%xd" or "%f".

Change-Id: Ia56cd6f5793a09fe5e19c91a8e8f9098f3244d57
Reviewed-on: https://cl.tvl.fyi/c/depot/+/3537
Tested-by: BuildkiteCI
Reviewed-by: sterni <sternenseemann@systemli.org>

users/sterni/nix/string/tests/default.nix

@@ -56,10 +56,17 @@ let
       }))
   ];
 
+  f = "f";
+  testPrintf = it "prints f" [
+    (assertEq "basic %s usage" "print ${f}" (string.printf "print %s" f))
+    (assertEq "% escaping" "100%" (string.printf "100%%"))
+  ];
+
 in
   runTestsuite "nix.string" [
     testTakeDrop
     testIndexing
     testFinding
     testMatch
+    testPrintf
   ]